Greater TSPO VT was significantly correlated with greater distress associated with preventing compulsive behaviors as reported on the Yale-Brown Obsessive Compulsive Scale (Y-BOCS). The single-nucleotide polymorphism rs6971 of the TSPO gene influences binding of second-generation TSPO positron emission tomography radioligands, including fluorine F 18–labeled N-(2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide. For the purposes of the display, corrected TSPO VT values for genotype are shown. For this, a linear model of TSPO VT = b0+b1*genotype was applied and b1 = 4.158 in the obsessive-compulsive disorder data set. Because the effect of genotype corresponded to a b1 value of 4.158, mixed-affinity binding (MAB) TSPO VT values were raised by 4.158 to visually correct them to high-affinity binding (HAB) TSPO VT values. Note, for this data set, this approach provided visually similar TSPO VT value corrections as are found with the other approach of multiplying TSPO VT from MAB cases by 1.4. For this polymorphism, high-affinity homozygotes are denoted as HAB and heterozygotes are denoted as MAB.

Marrie
RA, Fisk
JD, Tremlett
H,
et al; CIHR Team in the Epidemiology and Impact of Comorbidity on Multiple Sclerosis. Differences in the burden of psychiatric comorbidity in MS vs the general population. Neurology. 2015;85(22):1972-1979.PubMedGoogle ScholarCrossref

QuestionIs translocator protein distribution volume, a marker of the microglial component of neuroinflammation, elevated in the cortico-striato-thalamo-cortical (CSTC) circuit of obsessive-compulsive disorder (OCD)?

FindingsIn this case-control study, translocator protein distribution volume was significantly elevated in the CSTC of participants with OCD compared with healthy control individuals. The Yale-Brown Obsessive Compulsive Scale measure of distress associated with preventing compulsive behaviors was positively correlated with translocator protein distribution volume in the orbitofrontal cortex.

MeaningThe presence of neuroinflammation in the CSTC of OCD argues for extending investigations of serum autoantibodies to the entire CSTC and that immunomodulatory therapies should be investigated in adult OCD.

Abstract

ImportanceFor a small percentage of obsessive-compulsive disorder (OCD) cases exhibiting additional neuropsychiatric symptoms, it was proposed that neuroinflammation occurs in the basal ganglia as an autoimmune response to infections. However, it is possible that elevated neuroinflammation, inducible by a diverse range of mechanisms, is important throughout the cortico-striato-thalamo-cortical circuit of OCD. Identifying brain inflammation is possible with the recent advance in positron emission tomography (PET) radioligands that bind to the translocator protein (TSPO). Translocator protein density increases when microglia are activated during neuroinflammation and the TSPO distribution volume (VT) is an index of TSPO density.

Conclusions and RelevanceTo our knowledge, this is the first study demonstrating inflammation within the neurocircuitry of OCD. The regional distribution of elevated TSPO VT argues that the autoimmune/neuroinflammatory theories of OCD should extend beyond the basal ganglia to include the cortico-striato-thalamo-cortical circuit. Immunomodulatory therapies should be investigated in adult OCD, rather than solely childhood OCD, particularly in cases with prominent distress when preventing compulsions.

Introduction

Obsessive-compulsive disorder (OCD) is a debilitating neuropsychiatric disorder, significantly affecting the function of 1% to 2% of adolescents and adults.1 A third of OCD cases inadequately respond to pharmacotherapies with good evidence for this condition, such as serotonin reuptake inhibitors and clomipramine.2,3 Some directions, based on models of striatal dopaminergic hyperactivity with stereotypy and abnormal glutamate uptake/regulation in the cortico-striato-thalamo-cortical (CSTC) circuit, have led to some positive clinical trials of augmentation with antipsychotics and N-methyl-d-aspartate receptor antagonists, respectively.4,5 Although such findings are being incorporated in treatment algorithms,3 more consistently impactful treatment approaches are needed and the major barrier in therapeutic development is the paucity of novel pathological targets identified in the brain of OCD.

An autoimmune pathophysiology has been proposed for several case series of OCD characterized by prepubertal acute onset, episodic course, and concurrent neurological abnormalities such as choreiform movements occurring or exacerbating after exposure to infection.6,7 This syndrome has been termed pediatric autoimmune neuropsychiatric disorder associated with group A beta-hemolytic streptococcus (GABHS) (PANDAS) or pediatric acute neuropsychiatric syndrome (PANS). Cross reactivity between gangliosides in basal ganglia neurons with the GABHS cell wall is mechanistically implicated in Sydenham chorea, a series of rapid and uncoordinated jerky movements affecting the face, feet, and hands, which may occur within 6 months of the acute GABHS infection,8 and there are some reports of antibasal ganglia antibodies in the serum of PANDAS cases.9

Although PANDAS and PANS are relevant for only a minority of OCD cases,10,11 it is possible that the broader state of neuroinflammation and/or autoimmunity, inducible by a more diverse range of mechanisms, is important in OCD. Prevalence of anxiety disorder is strongly elevated, with rates of 30% to 40% in medical conditions such as systemic lupus erthymatosis and multiple sclerosis, for which autoimmunity is an important component of the medical disease.12,13 Also, specific prevalence rates of OCD in systemic lupus erythematosus and multiple sclerosis are typically several fold higher.14,15 In addition, OCD specifically is associated with developmental disorders such as Tourette syndrome and tic disorder for which excessive autoimmunity is also implicated.16 Moreover, induction of neuroinflammation in rodents with lipopolysaccharide administration is associated with anxiety behaviors, such as reduced exploration in the open field test, even when accounting for changes in motor activity.17,18

Microglial activation, an important component of neuroinflammation, can be measured with translocator protein (TSPO) positron emission tomography (PET) imaging because microglia increase expression of TSPO when they are activated.19 In a [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl]-3-isoquinoline carboxamide ([11C]PK11195) PET study of 17 children with PANDAS and 15 healthy control adults, Kumar et al,20 using a reference tissue approach, found elevated TSPO binding in the basal ganglia but not thalamic regions. These findings cannot be extrapolated as being applicable to OCD because this study investigated PANDAS rather than OCD. Also, this study restricted its regional analyses to the striatum and thalamus and did not investigate any other region implicated in the neurocircuitry of OCD. Given that with the exception of this neuroimaging study in PANDAS, which represents a small percentage of cases,10,11 there are no postmortem or neuroimaging studies of neuroinflammation in OCD, hence, it is not known as to whether neuroinflammation occurs in OCD.

While [11C]PK11195 was the first TSPO PET radiotracer, recent advances in PET radioligands for TSPO have resulted in a second generation of radiotracers for which the specific binding signal is at least several-fold higher, such as fluorine F 18–labeled N-(2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide) ([18F]FEPPA) PET.21,22 [18F]FEPPA is an excellent radioligand for TSPO, having high and selective affinity, considerable brain uptake, and good reversibility.23,24 Moreover, radioactive metabolites are not detected in brain24 and the binding of [18F]FEPPA is increased during induced inflammation in animals as well as the positive control condition of Alzheimer disease in humans, which is associated with microglial activation in response to amyloid deposition.25-27

The primary aim of this study is to compare TSPO distribution volume (VT) in the CSTC circuit of OCD with healthy control individuals. It is hypothesized that TSPO VT, being elevated in neuroinflammatory states, is greater in OCD. Because the strongest regional convergence across investigations of neurochemical abnormalities in OCD, such as 5-HT2A, 5-HTT, 5-HT1B, and mGluR5 receptor binding28-33 and fluorodeoxyglucose uptake,34 are in the dorsal caudate and the orbitofrontal cortex, these regions are given the highest priority. However, the neuropathology of OCD also implicates a broader group of structures in the CSTC circuit, such as the thalamus, ventral striatum, dorsal putamen, and anterior cingulate cortex,4,5,33,34 so these regions were given the next highest priority. Also, given that neuroinflammatory pathologies of the CSTC circuit, including vascular disease, tumors, Huntington disease, Tourette disorder, and Sydenham chorea, are associated with disturbances of complex motor behavior, the third aim, which was exploratory, is to assess the association between TSPO VT in the dorsal caudate and orbitofrontal cortex with severity of compulsions in OCD.

Methods

Participants

Twenty participants with OCD and 20 age-matched healthy control individuals (within 4 years) completed the study. Participants were recruited from the Toronto, Ontario, Canada, area at a tertiary care psychiatric hospital (Centre for Addiction and Mental Health) from May 1, 2010, to November 30, 2016. All were aged 19 to 48 years, nonsmoking, and in good physical health (see demographic characteristics in Table 1). None of the participants had a history of autoimmune disease, and all were free of medical illnesses for at least 4 weeks. Fourteen healthy control individuals (70%), reported in a previous study,35 were also included in the present study. No other healthy or OCD cases in this study were included in any other study. The remaining control individuals have not been included in other studies. Eleven participants (55%) had OCD prior to age 12 years and 18 participants (90%) had OCD onset prior to age 20 years. The presence or absence of psychiatric disorders and OCD specifically were confirmed using the Structured Clinical Interview of DSM-IV.36 Two OCD cases had a history of a major depressive episode, a single episode occurring 4 and 5 years prior to their scan date. The severity of OCD was measured with the Yale-Brown Obsessive Compulsive Scale (Y-BOCS).37,38 Clinical symptoms of OCD varied and severity of the obsessive-compulsive symptoms was moderate to severe on average (eTable 1 in the Supplement). None of the OCD cases met criteria for PANDAS or PANS.6,7

Participant diagnosis was verified by a psychiatrist (J.H.M. or L.R.). Exclusion criteria for all participants were pregnancy; herbal, drug, or medication (including psychotropic) use within the past 6 weeks, except for oral contraceptives; substance abuse (including cigarette smoking); and any history of neurologic illness or injury. Other exclusion criteria consisted of concurrent active Axis I disorders, current or past alcohol or substance dependence, bipolar I or II disorder, and borderline or antisocial personality disorder, the latter ruled out with the Structured Clinical Interview for DSM-IV Axis II disorders.39 All participants underwent urine drug screening, and women received a urine pregnancy test on the screening and PET scanning days. Participants provided written informed consent after all procedures were fully explained. The protocol and informed consent forms were approved by the research ethics board of the Centre for Addiction and Mental Health, Toronto, Ontario, Canada.

Image Acquisition and Analysis

Each participant underwent a single [18F]FEPPA PET scan conducted at the Research Imaging Centre at the Centre for Addiction and Mental Health. Intravenous [18F]FEPPA24 was administered as a bolus (mean [SD], 183.5 [10.5] MBq or 4.9 [0.3] mCi). The [18F]FEPPA was of high radiochemical purity (>98.0%) and high specific activity (mean [SD], 133.6 [118.7] TBq/mmol). The PET scan duration was 125 minutes after the injection of [18F]FEPPA. Positron emission tomographic scans were acquired using a 3-dimensional brain scanner (HRRT; CPS/Siemens). Positron emission tomographic scans were acquired and reconstructed as described previously.21,24,35,40,41 All PET images were corrected for attenuation using a single photon point source, cesium 137 (half-life, 30.2 years; energy, 662 keV), and were reconstructed using a filtered back-projection algorithm, with a Hann filter at Nyquist cut-off frequency.41 Manual and automatic blood sampling (ABSS, Model PBS-101; Veenstra Instruments) was conducted to determine the input function of parent compound in plasma for the kinetic analysis, based on the ratio of radioactivity in whole blood to radioactivity in plasma, and the percentage of parent compound in plasma.21 A biexponential function was used to fit the blood-to-plasma ratios and a Hill function was used to fit the percentage of unmetabolized tracer as previously described.21,35 A 2-tissue compartment model was applied to generate time activity curves from each region of interest (ROI) to quantitate TSPO VT with [18F]FEPPA PET, as this was previously demonstrated to be the optimal model for [18F]FEPPA PET.21

Regions of interest were automatically generated using the semiautomated software (ROMI; Research Imaging Centre, Centre for Addiction and Mental Health). ROMI is based on the individualization of a set of standard ROIs using magnetic resonance imaging (MRI) coregistered with the PET image, followed by a step of gray matter voxel selection, which incorporates the probability of gray matter based on the segmentation of the individual MRI as previously described.42,43 For the anatomical delineation of ROIs, the first 13 participants (OCD: n = 8; healthy: n = 5) underwent 2-dimensional axial proton density MRI acquired with a General Electric Signa 1.5-T MRI scanner (section thickness, 2 mm; repetition time, >5300 milliseconds; echo time, 13 milliseconds; flip angle, 90°; number of excitations, 2; acquisition matrix, 256 × 256; and field of view, 22 mm). The remaining 27 participants (OCD: n = 12; healthy: n = 15) underwent 2-dimensional axial proton density MRI acquired with a General Electric Signa 3-T MRI scanner (section thickness, 2 mm; repetition time, 6000 milliseconds; echo time, 8 milliseconds; flip angle, 90°; number of excitations, 1; acquisition matrix, 256 × 192; and field of view, 16.5 mm). A subset of participants were scanned with both MRI machines to rule out bias due to delineation with ROMI. The differences in MRI acquisitions resulted in near identical TSPO VT values and regional volumes in a sample undergoing both MRI (n = 12; r > 0.97; P < .001; J.H.M., unpublished data, May 2010-February 2014).

DNA Extraction and Polymorphism Genotyping

The binding affinity of [18F]FEPPA for TSPO is affected by a single-nucleotide polymorphism (rs6971; C→T) in exon 4 of the TSPO gene (NCBI Entrez Gene 706).41,44 Individuals with high-affinity binding (Ala147/Ala147) and mixed-affinity binding (Ala147/Thr147) account for more than 95% of the population in the greater Toronto area.35,45 The polymorphism rs6971was genotyped as described previously.41 Homozygotes for the low binding gene (Thr147/Thr147) were excluded from data analysis.

Statistical Analysis

To test the primary hypothesis, a multivariate analysis of variance (MANOVA) was applied with TSPO VT in the dorsal caudate, orbitofrontal cortex, thalamus, ventral striatum, dorsal putamen, and anterior cingulate cortex as the dependent variables, with diagnosis as the key independent predictor variable and genotype (rs6971 polymorphism) as an additional independent variable. Main effects were considered significant at the conventional P ≤ .05. Effects in each region, analyzed by univariate analysis of variance (ANOVA), were considered significant after Bonferroni correction at P ≤ .009.

To assess whether there was a global difference in TSPO VT between participants with OCD and healthy control individuals, a MANOVA was also applied with TSPO VT from all regions sampled included as the dependent variables, with diagnosis and genotype as independent predictor factors. The association of TSPO VT in the dorsal caudate and orbitofrontal cortex with time occupied, interference, distress, resistance, and control over obsessions and compulsions was assessed applying a partial correlation coefficient controlling for genotype. This was treated as a posthoc analysis. Statistical analyses were performed using IBM SPSS version 22.0.46

To investigate whether there was evidence for a pattern of TSPO VT elevation in OCD that differs as compared with the elevations found in major depressive disorder,35 a post hoc analysis was done comparing the relative TSPO VT values in the dorsal caudate and orbitofrontal cortex, regions most implicated in OCD, in relation to TSPO VT in the anterior cingulate cortex, a region implicated in major depressive disorder (eAppendix in the Supplement).

To our knowledge, this is the first study to investigate inflammation in the brain of OCD. The most prominent finding is greater TSPO VT throughout the CSTC circuit implicated in OCD including the orbitofrontal cortex, dorsal caudate, thalamus, dorsal putamen, ventral striatum, and, albeit to a slightly lesser extent, the anterior cingulate cortex. This finding was also present in the other gray matter regions of brain at a slightly lower level of magnitude and this regional distribution is consistent with a neuroinflammatory phenotype. These results have important implications for the pathophysiology of OCD and novel therapeutic treatment directions.

The best explanation for elevated TSPO VT is that it reflects neuroinflammation consequent to microglial activation. Across diverse models of cerebral artery occlusion, toxin exposure, and lipopolysaccharide administration, it is consistently demonstrated that the temporal course of elevations in TSPO levels are highly correlated with markers of microglial activation.47-49 The widespread distribution of elevated TSPO VT throughout gray matter is consistent with investigations of TSPO VT with more sensitive radioligands in diseases with microglial activation and more focal pathologies such as Alzheimer disease and stroke, which tend to find that no gray matter region is fully spared from an elevation of TSPO VT.26,27,50 This could be accounted for by diffuseness of initiating disease pathology; spread of microglial activating disease–associated molecular patterns and/or pathogen-associated molecular patterns; and most plausibly, paracrine effects of activated microglia.51 Even so, it is interesting that TSPO VT was elevated by more than 30% within the orbitofrontal cortex, dorsal caudate, thalamus, dorsal putamen, and ventral striatum. The involvement of the CSTC has important implications for the autoimmune etiology of OCD. Traditionally, investigations of autoantibodies in serum of OCD cases mostly focus on striatum, presumably because GABHS is associated with Sydenham chorea and the PANDAS mechanism is based on the epitope cross reactivity of the striatum with GABHS.6,7 Some investigators are beginning to evaluate serum autoantibodies to other parts of the CSTC circuit such as the thalamus,52 but our study argues that it is critical that such studies consistently pursue autoantibodies for the dorsal caudate and orbitofrontal cortex as well as the remainder of the CSTC circuit.

This study demonstrates that microglial activation occurs well beyond the initial, frequently childhood, onset of OCD, and the presence of activated microglia provides a useful opportunity to modulate their function as a therapeutic strategy. Although pharmaceutical development does not traditionally prioritize OCD, neuromodulatory treatments under development for other diseases associated with microglial activation, such as Alzheimer disease,53,54 might be repurposed toward OCD. Microglial activation may include components harmful to neurons and glia (termed M1 responses), such as creating reactive oxygen and nitrogen species and producing proinflammatory cytokines, but also include helpful components (termed M2 responses) such as clearing cellular debris, inducing angiogenesis, and promoting tissue repair.55 More specifically, the current study would suggest that medications under investigation for shifting activated microglia from an M1 to M2 state, such as azithromycin, minocycline, and bexarotene,56 should be investigated in OCD, particularly in samples in which the severity of distress associated with preventing compulsive behaviors is highly prevalent. Consistent with consideration of an immunomodulatory approach, minocycline, an antibiotic that, among its effects, inhibits major histocompatibility complex II expression and reduces M1 type markers,57 significantly reduced Y-BOCS scores in a recent randomized, double-blind, placebo-controlled study of add-on to fluvoxamine in 102 patients with OCD.58

Limitations

This study has several limitations, most of which are attributable to the interpretation of TSPO VT and the application of PET imaging. Elevated TSPO VT is well-established as a marker of microglial activation; however, given that TSPO has roles in translocating cholesterol from outer to inner cell membranes and may form an oligomer with the mitochondrial permeability transition pore,59 it is theoretically possible that as knowledge regarding TSPO increases, other factors will be identified that may influence TSPO binding in the brain. Also, binding of the PET radiotracer can be affected by changes in density and affinity of the target. In addition, the association between elevated TSPO and OCD, as well as greater distress associated with preventing compulsive behaviors, indicates significant associations among these phenomena but does not establish a causal one as this issue will require future study through a combination of longitudinal studies and assessment of immunomodulatory interventions.

Conclusions

In summary, to our knowledge, this study is the strongest evidence to date for inflammation in the brain in OCD. The demonstration of elevated TSPO VT in the CSTC circuit addresses a critical gap in the autoimmune/neuroinflammatory theory of OCD. The regional distribution of greater TSPO VT throughout the CSTC circuit argues for consideration of autoimmune mechanisms beyond the basal ganglia and suggests a new opportunity for repurposing immunomodulatory therapies to treat OCD.

Conflict of Interest Disclosures: Drs Wilson, Houle, and Meyer have received operating grant funds for other studies from Janssen, Bristol-Myers Squibb, Eli Lilly and Company, GlaxoSmithKline, Lundbeck, and SK Life Sciences in the past 5 years. Dr Meyer has been a consultant to Mylan, Lundbeck, Takeda, Teva, and Trius in the past 5 years. None of these companies participated in the design or execution of this study or in the writing of the manuscript. Dr Meyer is an inventor on 4 patents of blood markers to predict brain inflammation and/or to diagnose affective disorders and a dietary supplement to reduce depressed mood in the postpartum period. Dr Meyer is arranging collaborations with nutraceutical companies for the dietary supplement. No other disclosures were reported.

Funding/Support: This study was supported by a Canada Research Chair from the Canadian Institutes of Health Research (Dr Meyer), an operating grant from the Grant Family, and funding from the Canadian Foundation for Innovation and the Ontario Ministry for Research and Innovation (infrastructure).

Role of the Funder/Sponsor: The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; or decision to submit the manuscript for publication.

Meeting Presentations: This study was presented in part at the Society of Biological Psychiatry Annual Meeting; May 18, 2016; Atlanta, Georgia; and the American College of Neuropsychopharmacology Annual Meeting; December 6, 2016; Hollywood, Florida.

Marrie
RA, Fisk
JD, Tremlett
H,
et al; CIHR Team in the Epidemiology and Impact of Comorbidity on Multiple Sclerosis. Differences in the burden of psychiatric comorbidity in MS vs the general population. Neurology. 2015;85(22):1972-1979.PubMedGoogle ScholarCrossref